When Bnei Yisrael wanted to traverse Edom on their way to Israel, Moshe sent emissaries to the king of Edom saying, "So said your brother Israel: You know all the hardship that has befallen us. Our forefathers descended to Egypt ... and the Egyptians did evil to us." (Bamidbar 20:14-15)

There are two questions here:

Why did Moshe feel it necessary to relate the troubles of the past to the king of Edom?

"Your brother Israel. Why did Moshe mention brotherhood here?" (Rashi)

Chazal answer in Midrash Tanchuma as follows:

"You know all the hardship that has befallen us." [Moshe] said to [the king of Edom]: You know that when G-d said to Avraham, "Know with certainty that your descendants shall be aliens in a land not their own, and they will enslave them and they will oppress them" (Bereishit 15:13) -- we were enslaved and you were free ... This is comparable to two brothers, against whose grandfather a debt-document was presented. One of them paid it. Some time later, he began to ask something of his brother. He said to him, "You know that the debt that I paid was upon both of us, and I was the one who paid it. Therefore, do not deny the request that I ask."

Hashem said to Avraham: "To your descendents I will give this Land." (15:18) However, it is not clear who the descendents of Avraham are -- Yitzchak or Yishmael? Yaakov or Esav? And while it is possible to exclude Yishmael, since he is the son of the maid, Esav is different, as Malachi states: "Was not Esav the brother of Yaakov -- the word of Hashem -- yet I loved Yaakov." (Malachi 1:2) Why was Esav excluded and the promise fulfilled only with Yaakov?

This is because in the brit bein habetarim (covenant of the pieces), it says (Bereishit 15:13-18):

Your descendants shall be aliens in a land not their own, and they will serve them, and they will oppress them, four hundred years ... The fourth generation shall return here ... On that day Hashem made a covenant with Avram saying, "To your descendants I have given this land."

Thus, it is clear that the same descendants who will be aliens, and will descend to Egypt -- they are the very same descendants to whom the Land will be given.

Regarding Esav it says: "Esav took his wives, his sons, his daughters ... and went to a land because of his brother Yaakov." (Bereishit 36:6) Rashi cites a Midrash:

"Because of his brother Yaakov." Because of the debt of the decree, "Your descendants shall be aliens," which was placed on the descendents of Yitzchak. [Esav] said, "I will leave here, and I will share neither in the gift, that this land is given to him, nor in the payment of the debt."

Therefore, it says in the end of Parshat Vayishlach: "These are the chiefs of Edom by their settlements, in the land of their possession -- he is Esav, father of Edom" (36:43), and immediately afterwards it says: "Yaakov settled in the land of his father's sojourning" (37:1), and the story of the descent to Egypt begins. The account of the exile was fulfilled only through Yaakov, whereas Esav settled in his possessed land.

This is why Esav is mentioned in the Passover Haggadah: "To Esav I gave Mount Seir to inherit, and Yaakov and his sons went down to Egypt." (Yehoshua 24:4) Esav went complacently to his inheritance, while the decree of exile was fulfilled through Yaakov. Thus, they were the ones through whom the promise, "To your descendants I have given this land," was fulfilled.

This is why Moshe mentioned to the king of Edom, "So said your brother Israel." As Rashi explains, since we are brothers we should have both paid the debt, and therefore, "Let us pass through your land" -- you cannot contest the inheritance of Israel, just as you did not pay the debt.

Only the refinery of the suffering, subjugation, and the troubles of the exile, award rights to the Land of Israel, as Chazal teach: "G-d gave three good gifts to Israel, and all were given to them only through suffering: Torah, Eretz Yisrael, and the world to come."

With this we can understand Rashi's opening comment on the Torah in explaining the narrative of Bereishit. Rashi cites from the Midrash that it would have been proper to begin the Torah with, "This month shall be for you the beginning of the months" (Shemot 12:2), the first mitzvah given to Am Yisrael. However, the Torah begins with the story of Creation because of, "The strength of His deeds he declared to His people, to give them the heritage of the nations." (Tehillim 111:6) If the nations of the world will say to Israel, "You are robbers, for you stole the Land," we will say to them, "G-d created the world; He took Eretz Yisrael from them and gave it to us."

This interpretation does not explain, though, what is the purpose of all the stories from Parshat Lech Lecha, in which Eretz Yisrael is promised, and on? Rather, all the narrative until, "This month shall be for you," comes to teach how Yishmael and Esav were separated from the descendants of Avraham. Only through Yaakov was the decree of brit bein habetarim fulfilled, and only through him was the promise of the Land fulfilled.